Put the sugar and nothing else in a large heavy pan over med or slightly higher heat. Stir some and keep and eye on it while it starts to melt. When it begins to liquefy stir it steadily until completely turned to syrup and a nice amber color. Now pour in 3 cups of water, still on the heat, and lean back because it's going to suddenly look like the earth's crust with volcanic vents everywhere emitting steam! Just poke at it and stir and it will quickly liquify again. When it does, add 3/4 cup cocoa powder and 1/4 tsp salt and whisk till melted together. Cool completely and preferably refrigerate till really cold then add the vanilla. You can either churn it in the ice crm maker or just freeze it in a pan, but it's best for the eating when more of a soft serve consistency from the churn.